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    <title>Document</title>
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    <script>
        //原型对象的应用：扩展内置对象方法
        //1 查看Array的原型对象
        console.log(Array.prototype);
        //2 为原先的Array添加内置方法
        Array.prototype.sum = function() {
            var sum = 0;
            for (var i = 0; i < this.length; i++) {
                sum = this[i];
            }
            return sum;
        }
        var arr = [1,2,3];
        //3 调用新添加到Array的sum方法
        console.log(arr.sum());
    </script>
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